The present invention relates to an apparatus for optically connecting a single-mode optical fiber to a multi-mode optical fiber, and more particularly, to an apparatus for optically connecting a transmission side single-mode optical fiber to a receiving side multi-mode optical fiber without deterioration of a baseband transmission characteristic.
In an optical communication system, a light source, such as a laser diode ("LD") or a light-emitting diode ("LED"), must be changed in the field in accordance with its life-span. It is difficult to optically connect a light source to an optical fiber in the field because of the small diameter of the optical fiber. Therefore, a light core source is provided as a module which includes a short optical fiber already connected to a light source and fixed so that the light source can be easily connected to an optical fiber as a practical communication line with an optical connector, via this short optical fiber. This short optical fiber is called a pig-tail fiber.
For purposes of describing the invention, the optical fiber can be defined as a single-mode fiber ("SM" fiber) and a multi-mode fiber ("MM" fiber), depending on the transmission capacity. There are two types of MM fibers defined as a step-index fiber ("SI" fiber) and a graded-index fiber ("GI" fiber). For this description, the GI fiber will be discussed as representing a MM fiber. Two kinds of light sources are also commonly provided corresponding to the SM fiber and GI fiber. It is a recent trend that the light source be unified to the SM type light source and that this SM light source also be applied to existing GI fibers.
A coherent light source (e.g. an LD) is usually used for a single-mode optical fiber ("SM" fiber) communication line, so that a coherent light source has a SM fiber as a pig-tail fiber in order to be accorded a SM optical fiber communication line, and to be easily connected to the communication line.
On the other hand, to realize a higher speed optical communication, there is a recent trend to use a coherent light source rather than an incoherent light source (e.g. an LED) for both SM fiber and graded-index optical fiber ("GI" fiber) communication lines instead of replacing an existing GI fiber with an SM fiber.
In the case of using a coherent light source for an existing GI fiber communication line and for the purpose of being accorded optical fiber types, it has been considered to use a GI fiber for the pig-tail fiber. However, for the purpose of convenience and cost of manufacturing, it is better to use a SM fiber as the pig-tail fiber in common for both the SM fiber communication line and the GI fiber communication line.
Accordingly, because of the difference of fiber core L diameters, for example a GI fiber being 50 .mu.m and a SM fiber being 9 .mu.m, connecting a GI fiber comprising a pig-tail fiber to a SM fiber of a communication line is much more difficult than connecting a SM fiber comprising a pig-tail fiber to a GI fiber of a communication line.
As a result, it has recently been required to establish a technique for connecting a SM fiber of a pig-tail fiber, that is, in the transmission side, to a GI fiber of a communication line, that is, in the receiving side.
According to research, it has been found that in case an optical signal is input to a GI fiber from a SM fiber, the intrinsic transmission characteristics, i.e., the baseband transmission characteristic expressed by a 6 dB bandwidth, of the GI fiber deteriorates.
For a better understanding of this phenomenon, the 6 dB bandwidth will be explained with reference to FIG. 1 and FIG. 2.
FIG. 1 illustrates an attenuation of amplitude of an optical signal in an optical fiber transmission path. The amplitude A.sub.0 incident to the optical fiber 1 is attenuated to A at the output end of the optical fiber 1 and is then output.
In the case of transmission through an optical fiber, the higher the modulation frequency, the smaller the amplitude A of modulated waveform appearing in the output side, as shown in FIG. 2. This is due to three causes, such as mode dispersion, material dispersion and waveguide dispersion (structure dispersion).
Change of amplitude ratio (frequency characteristics) of input/output signals for modulation frequency beginning from OHz (Direct Current) is called a baseband transmission characteristic. Particularly the modulation frequency, shown as f.sub.1 in FIG. 2, at the point where the output amplitude A.sub.1 is lowered by 6 dB from the input amplitude A.sub.0 (0 dB) in a baseband characteristic is called a 6 dB bandwidth of an optical fiber, and is used as a measure to estimate frequency characteristics of an optical fiber.
A baseband characteristic of a GI fiber mainly depends on a modal delay time (mode dispersion) between modes propagated in the optical fiber. When the optical signal is input to a GI fiber from a coherent light source with a SM fiber as a pig-tail fiber (SM light source), an optical signal in the GI fiber is excited as a lower order mode. Therefore, it has been considered that the baseband characteristic is virtually improved more than the intrinsic 6 dB bandwidth of the GI fiber as much as a decrease in number of modes. In fact, when the distribution of refractive index of the GI fiber (hereinafter referred to as "profile") is ideal as shown in FIG. 3A, the 6 dB bandwidth is virtually improved. But, in case the profile of a fiber deviates from the ideal as shown in FIG. 3B, the 6 dB bandwidth deteriorates as described previously.
A difference in the distribution of refractive index in FIGS. 3A and 3B mainly depends on the method of manufacturing the optical fiber.
The optical fiber is manufactured by the following steps: a preform (mother material) is first made, and then the preform is heated, melted and finally spun into a fiber.
The internal CVD method (MCVD method) and external CVD method are well known as typical methods for making a preform.
In order to manufacture a fiber having the ideal profile shown in FIG. 3A, particular attention is required in the process of the internal and external CVD methods. Otherwise, the doping material, such as GeO.sub.2 molecules, escapes in the final "collapse" process and a dip region DIP (FIG. 3B) having a lower refractive index is generated at the core center area. In the actual manufacturing of optical fiber line, however, it is not cost effective to pay particular attention to this dip region DIP.
For resolving a deterioration of the 6 dB bandwidth of the fiber shown in FIG. 3B which has the profile deviating from the ideal one of FIG. 3A, certain corrective techniques have been proposed. Namely, a Step-Index optical fiber (SI fiber), having a large diameter and a length from several meters to several tens of meters long, is inserted between the pig-tail fiber of the SM light source and the GI fiber to enlarge an optical beam spot size output from a pig-tail fiber of the SM light source to match with the core diameter of the GI fiber. Alternatively, a lens system is inserted between the pig-tail fiber of the SM light source and the GI fiber to enlarge an optical beam output from the pig-tail fiber. These are further described in the Japanese Patent Publication Nos. 57-158604 and 62-78506.
However, according to recent research, it has been recognized that these prior art techniques cannot improve the 6 dB bandwidth of the GI fiber.